1. Field of the Invention
The present invention relates to a color cathode ray tube, and more specifically to an electron gun for a color cathode ray tube with an improved resolution over its entire phosphor screen.
2. Description of the Prior Art
FIG. 10 is a cross-sectional view of a shadow mask type color cathode ray tube, showing the structure of the color cathode ray tube of this kind. Designated 31 is a panel portion, 32 a neck portion, 33 a funnel portion, 34 a phosphor screen, 35 a shadow mask, 36 a mask frame, 37 a magnetic shield, 38 a suspension spring, 39 an electron gun, 40 a deflection yoke, 41 a correction magnetic device, 42 an inner conductive film, and 43 a high-voltage terminal.
In the color cathode ray tube shown in the figure, a vacuum enclosure is formed by the panel portion 31 having the phosphor screen 34 on its inner side, and by the neck portion 32 connected to the side-wall skirt portion of the panel portion 31 through the funnel portion 33. The neck portion 32 incorporates the electron gun 39.
The shadow mask 35 is fixedly supported on the mask frame 36 and is suspended by the suspension spring 38 on the inner side of the panel portion 31 so that the shadow mask 35 is close to the phosphor screen 34. The mask frame 36 is provided with a magnetic shield 37 for protection against external magnet fields.
Mounted over a transition area between the funnel portion 33 and the neck portion 32 is the deflection yoke 40 that deflects three electron beams Bc (a center beam) and Bs (two side beams) emitted from the electron gun 39 in horizontal and vertical directions. The deflected electron beams Bc, Bs then pass through the shadow mask 35 and land on the phosphor screen 34.
The phosphor screen 34 has a mosaic pattern of red, green and blue phosphor groups, each phosphor taking the form of a stripe or dot.
The shadow mask 35 is an electrode with a large number of apertures arranged so as to allow the three electron beams Bc, Bs to pass therethrough and precisely strike each of the three-color phosphor groups making up the phosphor screen 34, thereby performing a so-called color selection.
The funnel portion 33 has its inner wall coated uniformly with the inner conductive film 42 that extends to a part of the inner wall of the neck portion 32, and a high voltage is applied from the high-voltage terminal 43 piercing through the funnel portion. The funnel portion 33 is also coated with a conductive film on its outer wall.
The electron gun 39 includes a cathode of an electron beam generating section that produces, accelerates and controls three in-line parallel electron beams; a prefocusing section to control the electron beams; and a main lens section to converge the electron beams onto the phosphor screen 34.
FIG. 11 shows a distribution pattern of a magnetic deflection field generated by the deflection yoke. As shown in the figure, a horizontal deflection field 60 has a pincushion-like distortion and a vertical deflection field 61 a barrel-like distortion.
FIG. 12(a) and 12(b) show how the deflection field acts on an electron beam. A deflected scanning electron beam 62 in the periphery of the phosphor screen 34 receives not only a deflection force 63 as shown in FIG. 12(a) but also a horizontal diverging force 64 and a vertical converging force 65 as shown in FIG. 12(b), with the result that the beam spot on the phosphor screen 34 is deformed.
FIG. 13 shows the spot shapes of the electron beams landing on the phosphor screen. A central beam 62' on the phosphor screen 34 is circular, whereas electron beams 62" formed at the periphery of the phosphor screen 34 are deformed into non-circular shapes consisting of a high-luminance core portion 62"H and a low-luminance halo portion 62"L. A large vertical elongation of the halo portion 62"L in particular has adverse effects on the focusing characteristic. To reduce such degradation of the focusing characteristic, the conventional electron gun employs, for example, a construction disclosed in Japanese Patent Laid-Open No. 62-58549, in which a dynamic focusing voltage is applied to an electrostatic quadrupole lens.